1) Field of the Invention
The present invention relates to a magnetic recording and reproducing apparatus and, more particularly, to a magnetic recording and reproducing apparatus having a magnetoresistive head.
2) Description of the Related Art
In recent years, the recording density of a magnetic recording and reproducing apparatus has been extremely improved based on the employment of a head using the magnetoresistive effect. Particularly, along with practical use of a giant magnetoresistive head, the recording density has been improved significantly.
In general, a GMR (giant magnetoresistive) element consists of a free layer, a spacer layer, a pinned layer, and an antiferromagnetic layer. The pinned layer is magnetized in the element height direction, and the free layer is magnetized in the track width direction.
The magnetization direction of the free layer changes in the element height direction based on a change of a magnetic field of a magnetic recording medium. In this case, resistance becomes a minimum when the magnetization direction of the pinned layer and the magnetization direction of the free layer are mutually in parallel and also when both magnetization directions are the same. On the other hand, when both magnetization directions are opposite or anti-parallel, the resistance becomes a maximum. Therefore, information recorded on the magnetic recording medium can be read by detecting a change in the resistance.
It is most suitable that the magnetization direction of the pinned layer and the magnetization direction of the free layer of the GMR element form an angle of 90 degrees, when there is no external magnetic field. At this time, the output of the GMR element becomes a maximum, and positive/negative asymmetry of the output (i.e., asymmetry of a positive-direction output and a negative-direction output) becomes zero. However, according to GMR elements manufactured in an actual process, the angle formed between the magnetization direction of the pinned layer and the magnetization direction of the free layer is not 90 degrees. Because the magnetization state depends on the manufacturing process and the size of the GMR elements, there is a variance in the angle formed between the magnetization direction of the pinned layer and the magnetization direction of the free layer. Due to this variance, not all of a large number of GMR elements operate at an optimum bias point, and a predetermined maximum output cannot be obtained.
In this case, some characteristics can be adjusted by changing the sense current. In other words, the magnetization direction of the free layer can be slightly changed using a change in the magnetic field generated by the sense current. However, increasing the sense current causes an unstable characteristic, and this has a risk of bringing about disconnection due to electromigration. Consequently, it is difficult to control the head characteristics in actual practice.
Therefore, under the present situation, certain threshold values are provided for the output and the positive/negative asymmetry of a GMR head, and a head within this range of the threshold values is selectively used. However, according to this method, GMR heads cannot sufficiently exhibit a constant performance, because variance is permitted. This variance in characteristic also becomes a cause of poor yields in the manufacturing of the GMR heads. Further, even when a GMR head is in an optimum bias state at the beginning, the magnetized state of the pinned layer and the free layer will change due to various causes, such as heating, after the GMR head is mounted on a device. This may degrade the head characteristic and, further, generate a read error.
Conventionally, the bias is not adjusted from outside of the head to obtain an optimum characteristic during a data read operation. Japanese Patent Application Unexamined Publication No. 10-55501 discloses provision of a bias magnetic field generating element that is controlled according to head positional error information so as to optimize a positional relationship between a recording head and a reproducing head, thereby adjusting a sensitivity distribution of the reproducing head. However, in this publication, application of a magnetic field to the reproducing head to give a primary optimum bias to the reproducing head is not disclosed.